The printing process and the development processing operations have been shortened and speeded up with the photosensitive materials used for prints in recent years, and increasingly strong demands have arisen for higher speeds, increased processing stability and increased strength for handling purposes.
The most basic way of increasing the speed of a silver halide emulsion is to increase the grain size and thereby increase the amount of light which is adsorbed per grain. In the case of a dye sensitized emulsion, this is done in such a way that the light is absorbed by the dye such that the photoelectrons are transmitted to the silver halide and there is a link with the latent image formation. However, these methods do not always give satisfactory results, and increasing grain size impedes any speeding up of the development process. Also with dye sensitization, not only is development and desilvering impeded but, under normal conditions, there is little latitude for increasing speed by increasing the amount of dye sensitizing agent etc. Hence, the ability to increase the speed of silver halide grains without increasing the size of the grains and without increasing the amount of sensitizing dye would be very useful. The methods known as chemical sensitization methods are typical of the methods which are used for this purpose, and the known methods include those in which sulfur sensitizing agents such as sodium thiosulfate are used, those in which gold sensitizers such as potassium chloroaurate are used, those in which reduction sensitizing agents such as stannous chloride are used, and those in which these types of sensitization are used jointly. The speed which can be attained with these various methods of chemical sensitization is governed by the type of sensitizing agent and the amount which is added, and by the method used for making the addition and the composition, but the details have not yet been clarified, and it is known that different results arise according to the nature of the silver halide grains themselves prior to chemical sensitization. For example, it has been disclosed in "The Journal of Photographic Science", 14, 181 (1966) that differences arise with sulfur sensitization depending on the crystal habit of the silver halide grains, but it is noted on pages 249 to 256 of volume 23 (1975) of the same journal that the crystal habit of the grains, including reduction sensitization, may fulfill some role in latent image formation. Furthermore, the types of halogen from which the emulsion grains are formed, crystal habit, and the effect on the speed and fogging of sulfur sensitization and gold/sulfur sensitization on these materials have been discussed in "Photographic Science and Engineering" 28, 146 (1984). However, these reports only describe the effect of the nature of the silver halide emulsion grains on chemical sensitization and photographic speed and they give no indication of techniques or methods for responding to the demands for higher speeds and handling stability at the commercial level of the type referred to earlier.
Methods of realizing an effective increase in speed without increasing the silver halide grain size have been anticipated in the silver halide photographic materials as described above. When speed is increased in this way then further improvements in stability in processing and robustness in handling can also be anticipated.
The method of forming grains by so-called halogen exchange disclosed in JP-B-No. 50-36978 is one known method of increasing the sensitivity of silver halides. (The term "JP-B" as used herein means an examined Japanese patent publication.)
The silver halide emulsions formed using this method are characterized by having a higher speed and by having a reduced tendency to fogging as a result of the application of mechanical pressure. However, while the emulsions do have these distinguishing features, the inventors have discovered that they are accompanied by serious disadvantages. Thus, not only is the tendency towards fogging on the application of mechanical pressure reduced, but when parts to which pressure has been applied are exposed to light there is a marked loss of sensitivity. The extent of halogen exchange can be reduced in order to minimize this desensitization due to pressure, but in such cases fogging is liable to result from the application of pressure. Thus, there is a problem with both fogging and desensitization due to pressure and the two are incompatible. Moreover, it has been discovered that gradation is softened in halogen exchanged emulsions in which the extent of halogen exchange has been reduced.
Furthermore, the inventors have previously disclosed emulsions for which halogen exchange has been carried with mono-disperse emulsions using water soluble bromides in the presence of a cyanine dye, and when compared to the emulsions disclosed in JP-B-No. 50-36978, the emulsions obtained in this way were better in that they were excellent in respect of high speed, pressure desensitization and pressure fogging, but the development of a further technique was required to achieve more perfect pressure characteristics and high speed and high contrast.
Hence, this invention represents progress in this direction and it is intended to provide stable silver halide emulsions which have high speed and high contrast. In more practical terms, the invention is intended to provide a method for the manufacture of silver halide emulsions which, when chemically sensitized, have a high speed and high contrast and which have excellent pressure characteristics, and to provide silver halide photographic materials which contain these emulsions.